International Patent Applications WO98/31674 (=U.S. Pat. No. 6,103,718), WO99/31071, WO99/31090 and WO99/47505 (=U.S. Pat. No. 6,255,303) disclose phthalazinone derivatives having selective PDE4 inhibitory properties. In the International Patent Application WO94/12461 and in the European Patent Application EP 0 763 534 3-aryl-pyridazin-6-one and arylalkyl-diazinone derivatives are described as selective PDE4 inhibitors. International Patent Application WO93/07146 (=U.S. Pat. No. 5,716,954) discloses benzo and pyrido pyridazinone and pyridazinthione compounds with PDEIV inhibiting activity.